汉诺坝玄武岩Re-Os同位素地球化学--Re的挥发性丢失和壳-幔相互作用的证据

Re-Os isotope geochemistry of basalts from Hannuoba,North China:Evidence for Re volatile loss and crust-mantle interaction

本文报道了采自汉诺坝玄武岩区周坝和白龙硐剖面以及白布洛张20井等地29个玄武岩样品的Re、Os含量和^(187)Os/^(188)Os比值。Os含量为11×10^(-12)～314×10^(-12),Re含量为40×10^(-12)～238×10^(-12),Re和Os含量有正相关趋势。碱性玄武岩(AK)的Re、Os含量高于拉斑玄武岩(TH)和过渡玄武岩(TR),玄武岩Os含量变化与分离结晶作用有关,玄武岩的低Re含量与地面喷发的火山岩浆脱气过程中Re的挥发性丢失作用有关。玄武岩的^(187)Os/^(188)Os比值为0.14735～0.61136,AK的^(187)Os/^(188)Os比值比TH和TR低且变化小。玄武岩的^(187)Os/^(188)Os比值与Os含量有负相关性。随着Os含量降低到小于75×10^(-12),^(187)Os/^(188)Os比值迅速升高,反映了地壳混染在TH和TR成因中的贡献。在以往的研究中没有观察到类似的地壳混染作用,说明了Re-Os同位素体系在示踪壳源物质上的优势。一些Os含量较高的TH的^(187)Os/^(188)Os比值表明其地幔源区既非亏损的又非经交代富集的SCLM,可能是混入了地壳俯冲物质的"Marble cake"型地幔。总之,汉诺坝玄武岩的Re-Os同位素地球化学研究支持了以往研究的主要成果,两类玄武岩地球化学差异性和异源成因论;分离结晶和部分熔融过程在玄武岩成因中的重要作用;碱性玄武岩的成因与地幔柱的关系等。同时揭示了一些新的现象:汉诺坝玄武岩形成中存在少量的地壳混染作用;地面喷发的火山熔岩在脱气过程中Re的挥发性丢失;拉斑玄武岩的源区更有可能为"Marble cake"型地幔。

The whole rock Re-Os isotopic data of 29 basalt samples collected from Bailongdong and Zhouba sections as well as Zhang-20 drill,Hannuoba,North China are reported in this article.The results show that the Hannuoba basalts have relatively high Os abundances(11×10^(-12)～314×10^(-12)) and relatively low Re abundances(40×10^(-12)～238×10^(-12)) and show a positive correlation between them.Re and Os abundances of alkali basalt are relatively higher than that of tholeiitic and transitional basalts.The low concentration of Re in samples likely reflects volatile loss during magmatic outgassing in subaerial lavas.The abundances of Os are related to the indices of partial melting and fractional crystallization processes.The Os isotopic ratios(^(187)Os/^(188)Os) of the Hannuoba basalts range from 0.14735 to 0.61136.The alkali basalts have slightly radiogenic Os isotopic compositions within narrow range, whereas the tholeiitic and the transitional basalts have higher radiogenic Os isotopic compositions and wider range.A negative correlation is observed between Os isotopic compositions and Os abundances in Hannuoba basalts.The ^(187)Os/^(188)Os ratios raise rapidly as Os abundances decrease below 75×10^(-12),which reflects that the effects of crustal contamination on the petrogenesis of the basalts, especially for that of tholeiitic and the transitional basalts,and reflects Re-Os isotopic systematics are more powerful than Sr-Nd-Pb isotopes schemes for tracing crustal contamination in basalt petrogenesis.A few tholeiitic samples with rather high Os abundance have radiogenic Os isotopic compositions which probable indicates that tholeiitic basalts were derived from'marble cake'type mantle,not from depleted and metasomatic enriched SCLM.In a word,this study not only approves the primary conclusions in previous research of Hannuoba basalts,but also uncovers some new phenomena,such as Re volatile loss and manle-crust interaction.